Magnetic devices and preparation thereof



June 4, 1963 F. H. EDELMAN MAGNETIC DEVICES AND PREPARATION THEREOF 2Sheets-Sheet 1 Filed March 2. 1959 Om N o JOEPZOU R N Y T M N N L R E E0 V D T mm W. E M

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MAGNETIC DEVICES AND PREPARATION THEREOF Filed March 2, 1959 2Sheets-Sheet 2 FRANK H. EDELMAN ATTORNEY United States Patent "ice3,092,510 MAGNETIC DEVICES AND PREPARATION THEREOF Frank H. Edelman,Philadelphia, Pa., assignor to Sperry Rand Corporation, New York, N.Y.,a corporation of Delaware Filed Mar. 2, 1959, Ser. No. 796,681 15Claims. (Cl. 13.7-93.2)

This invention relates to magnetic devices, and more particularly tosquare hysteresis loop soft magnetic films with low anisotropy.

This invention further relates to novel methods for making magneticdevices, and more particularly to novel methods for making isotropicsquare hysteresis loop magnetic films.

This invention is a continuation-in-part of a co-pending applicationentitled Magnetic Devices and Preparation Thereof, Serial No. 775,038,filed November 19, 1958, by Frank H. Edelman.

In the past, anisotropic magnetic films have been produced byevaporating magnetic materials in an evacuated zone and condensing thevapors onto a substrate. Some of these films have square hysteresisloops. However, these films are anisotropic in character; that is, theyhave two (each one opposite to each other) easy directions ofmagnetization. Thus, the magnetic films of the prior art, generally,have two stable states.

By the teachings of this invention, magnetic films are produced with lowanisotropy, that is, the films produced by this invention havesubstantially isotropic magnetic characteristics. These films,therefore, have more than two easy directions of magnetization and aretherefore usable for multi-state operation. More particularly, whenisotropic films are used for two state operation, they are capable ofbeing switched from one state to the second orthogonal state at muchhigher speeds than other magnetic films of the prior art.

In accordance with the invention, as set forth in the above-identifiedco-pending application, metallic salts of a B-diketone are separatelyheated, and their vapors are carried by a carrier gas to a heatedsubstrate for deposition thereon. For example, iron acetyl acetonate isheated to a temperature of 130 C. and nickel acetyl acetonate is heatedto a temperature of 190 C. The vapors thereby produced are carried by areducing agent, such as hydrogen, to a substrate which has been heatedto 390 C. The temperatures may be varied, within certain limits, as setforth more fully hereinafter.

In accordance with the present invention, magnetic films are produced bythe use of a rotating magnetic field during the condensation of thevapors upon the substrate and/or during an annealing process. Althoughisotropy can be obtained with other types of magnetic fields, or with anabsence of a magnetic field, for best isotropy, the condensation of thevapors and/or annealing occurs in a rotating circular magnetic field.

It is, therefore, an object of this invention to produce a novelmagnetic device with fast switching time.

It is a further object of this invention to provide a multi-statemagnetic device with substantially zero anisotropy.

Another object of this invention is to provide novel magnetic films withlow coercivity.

3,992,510 Patented June 4, 1963 It is a further object of this inventionto provide novel magnetic films which have rectangular hysteresis loopsthat are substantially isotropic in character.

Another object of this invention is to provide novel methods forcarrying forth the above objects.

It is a further object of this invention to produce novel methods fordepositing magnetic films.

The novel features of this invention and other obejcts and advantagesthereof, together with its organization and method of operation, willbecome more apparent in the following description, when read inconnection with the accompanying drawings, in which:

FIG. 1 is an illustration of suitable apparatus for performing oneembodiment of this invention;

FIG. 1a is an illustration of the coupling of coils shown in FIG. 1;

FIG. 2 is a perspective view of a magnetic device produced by carryingforth the novel methods of this'invention;

FIG. 3 is a diagrammatic representation of the switching process for anisotropic device produced by this invention; and

FIG. 4 illustrates a representative hysteresis loop of a magnetic deviceproduced through the novel methods of this invention.

Referring to FIG. 1, there is shown an illustration of apparatus forproducing magnetic devices 10 such as illustrated in FIG. 2. A magneticdevice 10 comprises a substrate 12, which may be glass, metal or quartz,to which is adhered a metallic film 14. As illustrated, the device 10 issquare or rectangular in shape; however, other shapes including circularor elliptical are easily obtainable, and may be preferred for variousapplications.

FIG. 1 shows a tubular container 16, preferably made of a high softeningpoint glass, such as that sold under the trademark Pyrex. A pair ofchambers 18, 20, are located at one end of the container 16 for carryingindividual portions of melt. For example, the chamber 18 is a wire sieveadapted for carrying iron acetyl acetonate; the chamber 20 is a wiresieve adapted for carrying nickel acetyl acetonate. A third chamber 22houses a plurality of substrates 12 which are mounted on the levelsurface of a brass batt 24. Separate heating coils 26, 28, 30 surroundthe three chambers 18, 20', 22, respectively. The coils 26, 28, 30 areshown in cross-section for simplicity of illustration. Individualthermocouples 3'2, 34, and 36 for measuring temperatures are coupled tothe re spective chambers 18, 20, 22. The thermocouples 3 2, 34, and 36are, respectively, coupled to control units 38, 40, 42 which control theheaters 26, 28, and 30. At one end of the tubing 16, an inlet 41 isprovided for introducing dry hydrogen as a carrier gas. At the other endof the tubing 16, an exhaust 43 is provided wherein the exhausted gasesare passed through. These gases are then cooled by a cold trap 44. Thedry non-condensible gases then pass through a flash back trap 46 andthen are ignited in a gas burner 56. The gas trap 44 is suitablychilled; as for example, by liquid air, or by carbon dioxide in itssolid state form. The exhaust gases from the exhaust 43 may be disposedby other desirable means. The substrates 12 are placed in a rotatingmagnetic field 52 for producing magnetic devices 10 of betteruniformity.

The use of a magnetic field for producing thin magnetic films is old inthe This invention, however, is concerned with the use of a circularrotating magnetic field which lies in the plane of the films. Filmsproduced by such a rotating field exhibit greater uniformity, higherBr/B'm values, lower coercivity and greater isotropy than films producedwithout such a field.

Chart A, shown below, illustrates the properties of magnetic films whensubjected to a rotating field during annealing compared with filmsprepared without a magnetic field. The data shown are from actual tests.It is believed that the drop of 0.08 in Br/Bm for Sample No.

2 is negligible, and may be a measurement error.

CharzA N Direction of Magnetization 1 Thickness Com- Sample (Angposi-Easy Hard" Type 0. stroms) tion,

Percent He, Br/Bm He, Br/Bm (e.) (0e.)

l 3,100 80 9.0 0.86 9.0 0.86 No field. 1 3,100 80 5.4 0.95 4.2 0.94Annealed Rotating field. 2 2,500 79 11.2 0.99 12.0 0.99 No field 2 2,50079 7.2 0.96 6.8 0.96 Annealed Rotating field.

Chart B, shown below, contains data from actual tests which illustratethe properties of magnetic films when subjected to a rotating fieldduring deposition of the films compared with those films depositedwithout a rotating field. a

The nickel-iron acetyl acetonates are separately heated in the wiresieve containers 18, 20 whose temperatures are adjusted to give theproper composition of the metallic film 14. The iron is heated toapproximately 130 C.; the nickel is heated to approximately 190 0.; andthedecomposition tube surrounding the substrates is heated to 390 C.Hydrogen is passed through the inlet-41 and thusthrough the tube 16 atapproximately 6 liters per minute for a period of 20 minutes atatmospheric pressure. The process may also be operated with hydrogen atpressures lower than atmospheric. The hydrogen, which acts as a reducingagent, carries the iron and nickel vapors to the substrates. Thecomposition of the films can be varied along the length of the tube 16so that compositions are obtained ranging from pure nickel to pure ironwith thicknesses ranging from a few angstroms to 10,000 angstroms. Theexcess 'gases are cooled, burned, and dissipated. These excess gases arequite varied and may include, for example, mesityl V oxide, methylacetate, acetone, butene-l, carbon dioxide, propylene, propane, methane,carbon monoxide, hydrogen, acetyl acetone, water, and mesitylene. Theexact constituents of the gases may vary depending upon its temperatureand the time of its measurements. The films can be deposited in acircular rotating magnetic field. In

addition, or instead, the films can be annealed, subsequent todeposition on a substrate, by heating the films to a temperature of 500C. to 550 C. and then cooling. The annealing process improvescoencivity, the squareness ratio, and the Br/Bm ratio.

A reference to FIG. 4 clarifies the meaning of the terms squarenessratio and Br/Em ratio. The squareness ratio is the ratio of the fluxdensity at a drive of minus one-half of the maximum drive to the fluxdensity at maximum drive; as shown, the squareness ratio is Bs/Bm. TheBr/Bm ratio is the ratio of the flux density at remanence to the fiuxdensity at maximum drive.

The thin magnetic alloy films that are produced by this invention haveuseful magnetic properties for magnetic memories because of their veryhigh squareness ratios,

low coercivity and millimicroseconds switching time.

These films have many advantages over the prior art. For example, theapparatus, as described above, is relatively inexpensive when comparedto .the apparatus required for vacuum deposition of films. The processof this invention can be a continuous one, as desired, in that a seriesof glass slides or other substrates 12 can be passed continuouslythrough the gals streams. Furthermore, multi component alloy films aremade by the process of this invention at lower temperatures than thehigh boiling point of the metals as required by the vacuum depositionprocedure. The deposition of the metal in a hydrogen environment is madein a circular rotating magnetic field, in the plane of the films, toprovide the desired magnetic orientation. Annealing with a rotatingmagnetic field is done with the same apparatus as the film deposition.Pinhole free films are prepared by the teachings of this invention, thuseliminating the presence of pin-holes such as produced with vacuumdeposited films of comparable thickness. More particularly, the magneticproperties of the films are such that they have square loops indirections mutually perpendicular to each other. The metal deposit,therefore, is quite different from the crystal deposit obtained byvacuum deposition procedure.

Specifically, metallic sal-ts of fi-diketones are selected which, upondeposition, form metals or alloys which have magnetic properties. Theiron and nickel salts of acetyl acetouates, when heated, produce vapors,the quantity of which are easily proportioned by temperature control.

The rotating circular magnetic field 52, lying in the plane of thesubstrates 12, can be produced by known techniques. For example, asshown in FIG. 1, a cylindrical coil 53 provides a magnetic field lyingin the plane of its axis. The square helical coils 54a, 54b provide amagnetic field lying in the plane of their common axis. The applicationof alternating current to the coils S3 and 54a, 54b, wherein the currentthrough coil 53 leads or lags the current through coils 54a, 54b bycauses a uniform circular rotating magnetic field.

'Whether the current leads or lags is immaterial, since it does notmatter Whether the field rotates in a clockwise or counter-clockwisedirection. Either direction of rotation is suitable.

The coils 54a, 54b, as shown in FIG. 1a, are coupled so as to producefields in the same direction. Alternatively, if one of the coils 54a,54b were wound in the opposite direction to the other, then liketerminals of the coils should be coupled together to provide fields inthe same direction. The coils 54a, 5411 if desired, could be wired inparallel, instead of in series as shown.

A 60 cycle alternating current source in the order of 90 volts, isapplied in parallel to both sets of coils 53 and 54a, 5411. One of thecoils such as 54a, has a resistance 59 in series. The other coil 53 hasa variable capacitor 57 and resistance 58 in series. The resistances58,- 59 are matched to provide equal fields by both coils 53, and 54a,54b. The capacitor 57 is varied to provide a 90 displacement in thefields produced by the coils.

When deposition is made in the presence of a rotating circular magneticfield of suitable strength, for

example, 36 oersteds, which is applied during deposition and cooling,there result films which display very low anisotropy, showing orthogonalcoercivities which deviate less than oersted and in some cases arewithin ,4 oersted of each other. In combination with these properties,the films display a relatively high squareness ratio of at least 0.8,and, frequently, 0.98; coercivities range from 1.4 to 20 with a majorityin the range from 2 to 3.

The apparatus illustrated in FIG. 1 may be constructed in differentforms, as will be suggested to anyone skilled in the art. For example,one substrate may be used with various geometries to conform tothe bestpractice for preparing memory elements. A substrate can be pretreated toform conducting and/or insulating layers so that deposition of magneticfilms can be made upon them which will result in a completed memoryelement.

Although the minimum magnetostriction of these films occurs at 78%nickel in comparison with the 82% nickel obtained from structures of theprior art, the preferred composition is 70-75% nickel and 3025% ironwith approximate minimum and maximum limits of 65% nickel and 85%nickel, respectively.

The preferred thickness range is from 2,000 A. to 3,000 A.; however,1,500 A. is a minimum value below which the results tend to becomeerratic. The preferred position of the substrate in the magnetic fieldis in or near the center. The rotating circular magnetic field is a 60cycle field ofapproximately 30 oersteds at the substrate and is circularwithin an estimated 58%. The iron acetyl acetonate is volatized at 130C. and the nickel acetyl acetonate at 190 C. The vapor mix is swept by acurrent of hydrogen over the substrates which are held to 390 C. Afterformation of films, substrates are than cooled to 100 C. while stillsubjected to the rotating magnetic field.

The films produced by this latter process can be switched, i.e., themagnetization direction can be changed, at greater speeds than otherdevices known to the prior art. Furthermore, these films do not requireorientation initially in a cross field (i.e. a field perpendicular tothe switching field), as those systems of the prior art, to eliminatenoise, as they are isotropic. In addition, these films do not requireany secondary orientation of the pick-up and drive coils as necessitatedin some prior art magnetic device and preparation methods. Furthermore,film memories can be constructed with great economy over prior artmethods, because less operations are needed and because the rejection ofplates for disorientation is eliminated. It is, therefore, apparent thatthis invention improves the art of recording information with magneticfilms.

The optimum temperatures, as stated above are iron acetyl acetonate: 130C., nickel acetyl acetonate: 190 C. and the substrate: 390 C. Theoptimum temperature range for iron acetyl acetonate is 125 C. to 135 C.If the iron acetyl acetonate is heated to less than 110 C. or more than135 C., not enough iron acetyl acetonate vapors are carried over forcomposition or excessive decomposition occurs, respectively. The optimumtemperature range for nickel acetyl acetonate is 175 C. to 195 C. If thenickel salt is heated to less than 120 C., no effective film occurs; ifless than 175 C., a composition results which is not desired. If thetemperature is greater than 195 C. there is too much de composition. Theoptimum temperature range for the substrate is 380 C. to 400 C. If thetemperature were raised above 500 C., an amorphous deposit results. Ifthe temperature was in excess of 400 C. there is a possibility that theheat would be transferred back to the previous chamber. If heated below380 C., poor films are produced, not consistent in composition; under300 C., no film results. Variations, however, in the geometry of theapparatus may suggest minor temperature changes consistent with theteachings of this invention. In particular, it has been found that a oneinch diameter tube, 38 inches in length, is suitable for theapplications described. However, it is desired that this invention notbe limited to specific dimensions herein described.

Magnetic devices of the prior art were, generally, of the bi-stablestate variety, wherein a magnetic film could be magnetized in either oneof two opposite directions. As described herein, by using magnetic filmsthat are isotropic in directions mutually perpendicular to each otherfour stable states are attainable so that the film could be switchedfrom one stable state to a second stable state at a high rate of speed.However, by the inventive processes described herein using hydrogen as acarrier gas, and using a rotating circular magnetic field which rotatesat a uniform speed a completely isotropic magnetic film is obtained,which film offers unlimited possibilities for many uses. A magnetic filmwhich is completely isotropic, such as produced by this invention, has avery large number of stable magnetic states. Therefore, the magneticfilm can be switched from one stable state in one direction to a secondstable state in a direction having an acute angle with the onedirection. Thus,

extremely high switching speeds are attainable.

Although FIG. 1 and FIG. 2 illustrate rectangular shaped magneticdevices and FIG. 3 illustrates circular shaped magnetic elements, anydesired shape can be used. Preferably, a circular configuration is theoptimum shape.

FIG. 3 illustrates the switching procedure using an isotropic magneticdevice. The magnetic film has a remanent flux density B in the directionshown in FIG. 3a and has a magnetic field H applied at an angle thereto,as shown in FIG. 3b. The direction of the magnetic flux density Bchanges and becomes parallel to the direction of the magnetic field H(see FIG. 30). Upon the removal of the magnetic field H, the remanentflux density B remains in its position as shown in FIG. 3d. The magneticflux density changes its direction either by the wall migration process,or the domain rotation process, or a combination of both processes. Itis believed that, in most cases, especially with switching angles ofless than the domain rotation process occurs to cause the switching. Themagnetic flux density B remains in this latter position due to the factthat there is no easy or hard direction, but rather that all directionsare equally magnetizable in a purely isotropic magnetic device. Thisaction occurs only with magnetic devices which are substantiallyisotropic, or devices which are equally anisotropic in a plurality ofdirections. It should be noted that magnetic devices of the prior artare anisotropic and thus have only two stable states. When a magneticfield is applied perpendicular, in the same plane, to the remanent fluxdensity of a prior art anisotropic magnetic device, the vector B isdirected to its hard or unstable direction. The flux density, uponremoval of the magnetic field, would then resume an easy or stablestate, either to its initial condition or at a direction of from itsinitial direction.

The device of FIG. 3, can, by similar means, he switched back to itsinitial state by applying an appropriate magnetic field in the forwarddirection. It will be appreciated, therefore, that the magnetic film ofFIG. 3 can be switched from one state to a second state and back to itsfirst state again at much higher speeds than anisotropic devices of theprior art.

Novel methods are described for producing isotropic films. Also, novelmetallic films are produced which have characteristics not previouslyavailable in the art. These novel films have many advantages includingtheir low anisotropy; these films can be produced with great economy.

Having thus described this invention, it is desired that this inventionnot be limited to any specific embodiment described, but that thisinvention be defined by the scope of the claims.

7 What is claimed is: 1. A method of producing a substantially isotropicmagnetic device comprising heating at least one salt of fi-diketone anda magnetically susceptible metal to a temperature sufliciently high tovolatilize said metal salt but below the temperature at which the vaporsof the salt decompose, heating a substrate to a temperature not below380 C. and not greater than 500 C., passing a nonoxidizing carrier gasover the metal salt whereby vapors of the metal salt are admixed withthe carrier gas and contacting said gaseous mixture withthe 'heatedsubstrate for a sufiicient period of time to plate the substrate with acoherent isotropic film substantially free from pin holes, and applyinga magnetic field to said substrate during the plating thereon of theisotropic film.

2. A method according to claim 1 in which the field is a rotatingmagnetic field.

3. A method according to claim 2 in which the metallic salts are saltsof nickel and iron and they are heated to temperatures to produce amixture of vapors which on contact with the heated substrate produce afilm having from 65 to 85 percent nickel.

4. A method according to claim 3 in which the proportions of vapors ofiron and nickel salts on contacting the heated substrate produce a filmhaving from 70 to 75 percent nickel.

5. A method according to claim 2 in which the substrate is heated to atemperature between 380 and 400 C.

6. A method according to claim 2 in which the salts areacetylacetonates.

7. A method according to claim 6 in which the acetylacetonates areacetylacetonates of nickel and iron.

8. A method according to claim 7 in which the temperature to which theiron and nickel acetylacetonates are heated are adjusted so that thevapors carried by the carrier gas on contacting the heated substrateproduce a film having from 65 to 85% nickel.

9. A method according to claim 8 in which the temperatures are adjustedso that after contacting the substrate a film is produced having 70 to75 percent nickel.

'10. A. method according to claim9 in which the substrate is heated to atemperature from 380 to 400 C.

11. A method according to claim 8 in which the contact of the vapors ofiron andnickel acetyl'acetonates in the carrier gas withheated'substrate is suflicientlylong to produce a film of at least 1500angstroms.

12. A method according to claim 8 in which the carrier gas is hydrogen.

13 A method according to claim 8 in which the iron acetylacetonate isheated to a temperature from to C. and the nickel acetylacetonate isheated to a temperature fr0n1 to C.

14. A method according'to claim 8- in which the substrate is glass.

15. A method according to claim 8 in which the strate is quartz.

sub-

References Cited in the file of this patent UNITED STATES PATENTS1,675,120 Marden June 26, 1928 2,318,011 Parsons et al. May 4, 19432,430,520 Marboe Nov. 11, 1947 2,698,812 Schlad-itz Jan. 4, 19552,780,553 Pawlyk Feb. 5, 1957 2,785,651 Pawlyk Mar. 19, 1957 2,808,345Traub Oct. 1, 1957 2,877,138 Vodonik Mar. 10, 1959 2,898,227 DrummondAug. 4, 1959 2,900,282 Rubens Aug. 18, 1959 2,919,207 Scholzel Dec. 29,1959 FOREIGN PATENTS 519,584 Germany Mar. 2, 1931 572,409 Great BritainOct. 8, 1945 670,993 Great Britain Apr. 30, 1952 OTHER REFERENCESPreparation of Thin Magnetic Films and Their Properties, Blois, Journalof Applied Physics, vol. 26, No. 8, August 1955, pp. 975-980, page 980relied on.

1. A METHOD OF PRODUCING A SUBSTANTIALLY ISOTROPIC MAGNETIC DEVICECOMPRISING HEATING AT LEAST ONE SALT OF B-DIKETONE AND A MAGNETICALLYSUSCEPTIBLE METAL TO A TEMPERATURE SUFFICIENTLY HIGH TO VOLATILIZE SAIDMETAL SALT BUT BELOW THE TEMPERATURE AT WHICH THE VAPORS OF THE SALTDECOMPOSE, HEATING A SUBSTRATE TO A TEMPERATURE NOT BELOW 380*C. AND NOTGREATER THAN 500*C., PASSING A NONOXIDIZING CARRIER GAS OVER THE METALSALT WHEREBY VAPORS OF THE METAL SALT ARE ADMIXED WITH THE CARRIER GASAND CONTACTING SAID GASEOUS MIXTURE WITH THE HEATED SUBSTRATE FOR ASUFFICIENT PERIOD OF TIME TO PLATE THE SUBSTRATE WITH A COHERENTISOTROPHIC FILM SUBSTANTIALLY FREE FROM PIN HOLES, AND APPLYING AMAGNETIC FIELD TO SAID SUBSTRATE DURING THE PLATING THEREON OF THEISOTROPIC FILM.